CALPAIN MEDIATES EUKARYOTIC INITIATION-FACTOR 4G DEGRADATION DURING GLOBAL BRAIN ISCHEMIA

Global brain ischemia and reperfusion result in the degradation of the eukaryotic initiation factor (eIF) 4G, which plays a critical role in the attachment of the mRNA to the ribosome. Because eIF-4G is a subst rate of calpain, these studies were undertaken to examine whether calp ain I activation during global brain ischemia contributes to the degra dation of eIF-4G in vivo. Immunoblots with antibodies against calpain I and eIF-4G were prepared from rat brain postmitochondrial supematant incubated at 37 degrees C with and without the addition of calcium an d the calpain inhibitors calpastatin or MDL-28,170. Addition of calciu m alone resulted in calpain I activation (as measured by autolysis of the 80-kDa subunit) and degradation of eIF-4G; this effect was blocked by either 1 mu mol/L calpastatin or 10 mu mol/L MDL-28,170. In rabbit s subjected to 20 minutes of cardiac arrest, immunoblots of brain post mitochondrial supernatants showed that the percentage of autolyzed cal pain I increased from 1.9% +/- 1.1% to 15.8% +/- 5.0% and that this wa s accompanied by a 68% loss of eIF-4G. MDL-28,170 pretreatment (30 mg/ kg) decreased ischemia-induced calpain I autolysis 40% and almost comp letely blocked eIF-4G degradation. We conclude that calpain I degrades eIF-4G during global brain ischemia.